The scintillation proximity assay (SPA) is an approach for assay development and biochemical screening that allows rapid and sensitive measurement of molecular interactions in a homogeneous system, obviating the need for separation and washing steps. In theory, all that is required in the assay is mixing and measurement. The technology has proven useful in radiometric screening since its adoption in about 1992, with hundreds of PubMed literature references citing SPA applications to date. SPA is considered to be convenient, cost effective and safer than radioactive filter binding assays, providing fewer handling steps, no need for filters and scintillation cocktails as well as reduced disposal costs. The signal detection for SPA can be performed using any photomultiplier tube-based scintillation counter or CCD camera imager. SPA has enabled advances in high throughput screening, being both automation-friendly and requiring minimal hands-on involvement. It has been estimated that the assay provides a 30-fold increase in productivity relative to typical filtration assays.
Turning to the technical aspects of SPA, binding reactions can be assayed without the washing or filtration procedures normally used to separate bound from free fractions. Assays are typically performed using radioactive labels that emit electrons with only a short range (about 10 um) in water. When bound close to a solid scintillator surface by the binding reaction the radioactive labels are able to transfer electron energy to the scintillator to produce photons detectable with a scintillation counter. Electrons emitted from labeled molecules not bound close to the surface dissipate their energy in the medium and are not detected. The amount of light (photons) generated is proportional to the amount of radiolabeled molecules bound to the solid scintillant. Thus the bound fraction is detected specifically without separation of the solution from the support.
SPA beads are microscopic beads which contain a scintillant that can be stimulated to emit light. As is indicated above, this stimulation event only occurs when radiolabeled molecules of interest are bound to the surface of the bead, then blue light is emitted that can be detected on standard scintillation counters. Another type of SPA beads, often referred to as SPA imaging beads, emit red light that can be detected on standard CCD cameras. Assay plates coated with scintillant have also been used for SPA methods.
Further applications, formats, materials and procedures for performing SPA and SPA-like technologies are expected to contribute further to high throughput screening capability as well as to advances in bioanalytical and biomedical sciences.